The proposed study is designed to provide information on the role of the differentiated surfaces of the plasma membrane in the regulation of transepithelial salt and water transport. Previous studies have assigned distinct and key functions to the apical plasma membrane, a rate-limiting step for entry of salt and water into the epithelial cell, and to the basal-lateral plasma membrane, the putative site of the sodium pump. Reagents that bind covalently to surface groups will be used to label each of the epithelial surfaces. Differential labeling of the apical and basal-lateral plasma membranes would facilitate attempts to separate and characterize these surfaces. A clearer definition of the basic components of epithelial transport systems would contribute to our understanding of the mechanisms of action of hormonal regulators, e.g., aldosterone and vasopressin, and to disease states characterized by abnormalities in epithelial transport, e.g., nephrotic syndromes, chronic renal failure, congestive heart failure and hypertension.